The present disclosure relates to a catadioptric lens system that has a long focal length and is suitable for still image shooting and video shooting, and an image pickup unit using such a catadioptric lens system.
It is generally known that catadioptric lens systems configured of a reflective system and a refractive system are highly advantageous in reducing an entire lens length and reducing chromatic aberration, and have a configuration suitable for a long-focus lens. While typical focusing modes of the catadioptric lens systems include modes of varying the entire lens length, such as a unit focusing mode and a mode of changing a space between two reflection surfaces, there are inner focusing mode catadioptric lens systems in which an entire optical length is invariable. For example, Japanese Unexamined Patent Application Publication No. S58-205124 discloses a catadioptric lens system including a first lens group that includes a reflecting member and has positive refractive power, a second lens group that is movable along an optical axis and has negative refractive power, and a third lens group that has positive refractive power, the first to third lens groups being provided, in order, on a light path of incident light and in a direction of travel of the incident light. In the catadioptric lens system disclosed in Japanese Unexamined Patent Application Publication No. S58-205124, an afocal system is formed with use of the first lens group and the second lens group in an infinity focusing state, and traveling of the second lens group toward an image-plane side brings the catadioptric lens system into focus on an object at a short distance.
On the other hand, in recent years, there have been provided so-called single-lens mirrorless cameras that are single-lens reflex cameras without quick return mirrors. Since the quick return mirrors are removed, camera bodies are downsized, and convenience of portability is remarkably improved. Since, unlike the single-lens reflex cameras, in the single-lens mirrorless cameras, light constantly reaches an image pickup device for image pickup, the single-lens mirrorless cameras are suitable for high-quality video shooting. Moreover, a so-called wobbling technique has been generally known. In the wobbling technique, to keep focusing on an object moving back and forth with respect to a camera during video shooting, a lens is minutely vibrated, and is moved in a direction where high contrast is obtainable. To allow a focus group to perform a wobbling operation, it is preferable that the focus group have the inner focusing mode as a focusing mode, and be configured to be as small in size and weight as possible. When the size and weight of the focus group are reduced, a reduction in the size of an entire lens including a focus driving mechanism is achievable, and a reduction in power consumption caused by focus driving is also achievable.